Porous hollow fiber membranes are used in various fields, such as water purification treatment or waste water treatment by membrane filtration, or dehumidification or humidification.
Water purification treatment or waste water treatment by membrane filtration has recently been used widely in the field of water treatment because the maintenance and monitoring of operations are easier, and the quality of treated water is better, as compared with exiting filtration systems with flocculation and sedimentation. For example, membranes used in the membrane separation treatment of a membrane reactor method [MBR] combining activated sludge treatment and membrane separation treatment are required to have high strength, durability, and chemical resistance. Accordingly, the polyvinylidene fluoride [PVDF] membranes prepared by a thermally induced-phase separation method disclosed in Patent Documents 1 and 2 are often used.
However, such PVDF membranes prepared by a thermally induced-phase separation method have a strength of about 8 to 22 MPa. Further, among these, many membranes in practical use have a strength of about 11 MPa, which is a certain level of strength, but do not always have sufficient strength, as compared with membranes prepared by a non-solvent induced-phase separation method. Moreover, the thermally induced-phase separation method has complicated steps, and requires washing with many solvents. Thus, this method causes high cost and is hardly eco-friendly.
In contrast, membrane modules (membrane area: about 10 to 100 m2) having a structure in which polysulfone or PVDF and the like prepared by a non-solvent induced-phase separation method is fixed in a resin case using an adhesive are also often used in waste water treatment and water purification treatment. Such a membrane module is used such that water is supplied thereto in an amount of several 10 L to several 100 L per minute. In such a case, chemical washing or swinging washing is regularly performed to recover the flow rate; thus, the hollow fiber membrane may be broken during use or washing.
Furthermore, a method for performing dehumidification or humidification by a hollow fiber membrane system has many advantages, such as no need for maintenance and no need for a power source for driving. As such dehumidifying membranes or humidifying membranes, film-forming resin materials such as polyimide, polysulfone and polyphenylsulfone are used (e.g., Patent Document 2). Dehumidifying membranes using such materials are used in many industrial fields; however, due to their porous properties, the membranes have low absolute strength. Depending on the usage, the membranes are used under flow of a large amount of gas; therefore, the hollow fiber membranes may be broken during use. On the other hand, many humidifying membranes have recently been used to humidify the diaphragms of fuel cell stacks; however, in this case, a large amount such as about 4,000 NL/min of air flows, for example, for use in vehicle, and this may cause breakage of the hollow fiber membranes in relation to the mechanical strength.
As the method for increasing the mechanical properties of porous hollow fiber membranes, there is a method to increase the thickness of the hollow fiber membranes; however, this method is not preferable because it reduces the permeability of the hollow fiber membranes. Moreover, Patent Document 3 proposes a hollow fiber membrane obtained by forming a porous membrane layer in the outer periphery of a hollow support, such as a braid or knitted braid. In this method, the thickness of the hollow support is generally as thick as about 2 mm, which results in the inner diameter of the obtained hollow fiber membrane being larger than the thickness of the hollow support. Thus, in order to obtain the same membrane area as that when no hollow support is used, it is necessary to increase the volume of a module in which bundled hollow fiber membranes are stored. Further, since the permeation of an object to be treated occurs only in a porous layer portion placed in the gap of the hollow support, the permeability of the entire membrane may be reduced.
In contrast, Patent Document 4 proposes a porous membrane comprising a reinforcing fiber embedded in the membrane. In this method, the diameter of the hollow fiber membrane can be reduced to about 0.5 to 1.5 mm by setting the diameter of the reinforcing fiber to about 10 to 300 μm. In addition, the permeability of an object to be treated and the like is not reduced in a portion of the obtained hollow fiber membrane in which the reinforcing fiber is not embedded. Thus, the permeability of the entire membrane can be increased.
However, in some porous hollow fiber membranes obtained by this method, the reinforcing fiber may be embedded in a functional layer that largely affects the separation performance of the hollow fiber membrane, sometimes causing a significant reduction in the separation performance. For example, in soft materials such as polyester that are generally used as reinforcing fibers, when the gaps between the fibers (distance between the single fibers) are widened, meandering is likely to occur, and the reinforcing fiber may be embedded in the functional layer that largely affects the separation performance of the hollow fiber membrane. As a result, the mechanical properties such as strength and tensile elastic modulus of the hollow fiber membrane are likely to decrease. Therefore, the fiber gaps are reduced; however, air present in the narrower gaps can hardly be removed, and is likely to remain as a void of the hollow fiber membrane, which may cause poor impregnation properties.